1. Field of the Invention
This invention relates to a novel polysilane-polysilazane copolymer (copolymer) and methods for its preparation and use. The copolymer can be formulated into a composition. The copolymer is useful to prepare gap filling thin films or for casting fibers. The thin films can fill a gap with a width ≦100 nanometers (nm) and an aspect ratio (A/R)≧6.
2. Background
Shallow Trench Isolation (STI) can be used to achieve proper isolation between devices such as transistors in integrated circuits (ICs). STI involves forming trenches in a semiconductor substrate, and then filling the trenches with an insulating material. These filled trenches define the size and placement of the active regions. The pre-metal dielectric (PMD) layer on an IC isolates structures electrically from metal interconnect layers and isolates them electrically from contaminant ions that degrade electrical performance. PMD layers may require filling narrow trenches having a high A/R. Insulating material is deposited into the trenches to form barrier layers and planarize the topography. Chemical vapor deposition (CVD) and spin-on glass deposition (SOD) are techniques typically used to fill trenches on semiconductor substrates to form dielectric layers such as silicon dioxide (SiO2) and silicon dioxide-based layers.
A typical CVD method involves placing a substrate in a reactor chamber where process gases are introduced and heated. This induces a series of chemical reactions that result in the deposition of a desired layer on the substrate. CVD methods can be used to prepare a silicon dioxide film made from, for example, silane which has the formula SiH4 or tetraethoxysilane which has the formula Si(OC2H5)4. Boron and phosphorus doped silicon glass (BPSG) deposited by CVD is also a PMD material used in CVD methods. The BPSG films are first deposited in a CVD chamber and thereafter are furnace annealed at 850° C., which is above BPSG reflow temperature, to eliminate voids and improve planarization. However, with the dimensions of IC features quickly approaching nano-scale, the filling of these narrow, high aspect ratio (A/R≧6) trenches with void-free BPSG becomes difficult even after a high temperature annealing. The stringent gap fill of PMD becomes even more challenging as the industry strives to process PMD films at a lower annealing temperature, for instance, at 700° C. Various types of CVD processes are known, such as atmospheric pressure CVD, low pressure CVD, or plasma enhanced CVD. However, CVD methods may suffer from the drawback that when trench dimensions approach nano-scale, sufficient trench filling becomes difficult, particularly with dynamic random access memory (DRAM) devices, due to the voids formed during the process. The CVD techniques, therefore, are unsuitable for filling trenches with narrow widths and high A/R.
In a typical SOD method, a solution containing a film-forming material, such as a methylsilsesquioxane (MSQ) resin or a hydrogensilsesquioxane (HSQ) resin, is deposited on a spinning substrate to form a uniform thin film. The spinnability of the solution directly influences the quality and performance of the thin film. After the film-forming material is deposited on the substrate, the film-forming material is cured. Good trench filling properties can be obtained if the cured film-forming material in the trench can resist HF wet etching. To achieve resistance to HF wet etching, the cured material in the trench must be dense and/or hydrophobic to afford an HF etching rate comparable to thermal silicon dioxide. In the SOD process, it is known that MSQ resin can fill the small gaps with good etch resistance due to the introduction of hydrophobic carbon. However, because both STI and PMD applications are in the most sensitive regions of a semiconductor chip, severe electrical problems, such as leakage, can be caused by carbon. Thus, IC manufacturers, particularly dynamic random access memory (DRAM) device manufactures, are interested in finding a carbon free SOD material solution for trench filling applications.
HSQ resin has been considered as a candidate for trench filling applications. However, HSQ resin may suffer from the drawback that without the carbon of the MSQ resin, the thin film formed after cure in nano-scale trenches having high aspect ratios has insufficient density resulting in insufficient HF wet etch resistance. The narrow width and high A/R of a given nano-scale trench may prevent the HSQ resin from forming a dense film at the bottom half of the trench during a high temperature cure, e.g., a target cure condition for PMD applications may be heating at 700° C. in steam for 30 minutes. The SiO2 films derived from spin-on HSQ resins may suffer from the drawbacks of being either severely damaged or completely etched away when exposed to HF etchants in the trenches. This unsatisfactory wet etch resistance is likely the result of inhomogeneous densification in the trenches or due to the formation of low-density regions during conversion to SiO2. In highly confined geometries, such as narrow, high A/R trenches, the density of the cured film is influenced by several factors including the degree of shrinkage, adhesion to the trench walls, moisture diffusion through the film, and temperature. A HSQ resin film that is simultaneously cured and annealed tends to form a dense skin on the surface of the trench, which limits mass transfer to the bulk film, especially the corners of the bases of the trenches. This results in less densification in these areas that, as a result, are much more susceptible to HF etch damage. An HSQ solution formulated with colloidal silica particles and binder technology was developed to increase the gap fill density, which showed improved etch resistance. However, solution stability and porous appearance made this formulation impractical to use. An ozone cure chemistry was applied to a low molecular HSQ formulation. The trench densification was achieved and the cured HSQ resin in the nano-scale gaps demonstrated good etch resistance. However, ozone cure is not well accepted by IC manufacturers. Similarly, a hydrogen peroxide and ammonium hydroxide combination approach was used to cause low temperature oxidation of HSQ resin in a trench. However, the stability and safety of peroxide solution are major concerns to IC manufacturers. Alternatively, nitrous oxide (N2O) can be used to cure HSQ resin films, resulting very low etch rate in a 200:1 HF solution. However, for nano-scale trenches, this method suffers from the drawback of yielding etch resistance only on the top part of a trench.
Conventional polysilazanes (PSZs) have also been evaluated by a number of IC companies. Conventional PSZs have been prepared by amination of mixtures containing methylchlorosilanes and methylchlorodisilanes with ammonia, primary amines, or secondary amines. Alternatively, conventional PSZs have been prepared by Wurtz-type co-condensation of dichlorosilanes that contains Si—N—Si bonding. This synthesis of PSZs can be achieved by co-feeding alkyl substituted dichlorosilane and 1,3-dichlorodisilazane to a sodium suspension. The isolated linear PSZ was clear and mobile, and had a weight average molecular weight Mw of 2,500. Each silicon atom in this PSZ was bonded to at least one methyl group. Conventional PSZs may suffer from the drawbacks of being expensive and having poor stability. Certain PSZs also suffers from the drawback of containing SiC bonds, which provide a source of undesirable carbon. The conventional PSZs are not proper precursors for SiN, SiON or SiO2 ceramic films that have been widely used in semiconductor devices as dielectric or barrier materials because these PSZs may suffer from the drawbacks of being difficult to process into films due to a low crosslinking degree, or could have the problems of metal contamination or of incorporation of Si—C bonds that are difficult to remove by most film processing methods.
Therefore, there is a continuing need in the IC industry to provide a film forming material that can fill trenches with narrow widths and high aspect ratios without voids in the trenches, and that form dense films when the materials are cured.